A paradigm for skeletal strength homeostasis.

This article integrates engineering principles with skeletal biology to describe skeletal strength homeostasis. Skeletal strength revolves around its perceived mechanical usage. Mass, geometric properties, and fatigue damage burden are the principle determinants of structural strength. Bone cells form sensor and effector systems that monitor usage and adjust strength and stiffness by changing mass, geometric properties, and fatigue damage burden. The bone lining cell-osteocyte complex is the sensor; the bone modeling and remodeling systems are the effectors. Deformation and fatigue damage in bone are the signals received by the sensor. Accumulated energy in the sensor's cytoskeleton determines the rate at which the sensor sends messages to the effectors. The activity of both effector systems is proportional to the rate of incoming messages. Modeling raises bone strength and stiffness by improving geometric properties as it adds bone where customary deformation is greatest. Remodeling improves bone strength by replacing fatigue-damaged areas without mass changes. Bone removed during modeling and remodeling comes from sites where the impact on bone strength and stiffness is least. Hormones and agents alter the rigidity of the cytoskeleton and, thus, its capacity to deform and store energy. Osteopenic agents make it more rigid, causing detection of fewer deformations and transmission of fewer loading signals to the effector. Osteotropic agents decrease the rigidity of the cytoskeleton, causing detection of more strain events and transmission of more loading signals to the effector. Agent treatment thus establishes false conditions of disuse or hyperuse.